Free Atomic Spectra MCQs with Answers

• A. a continuous band of all wavelengths
• B. a set of sharp, discrete lines at fixed wavelengths
• C. a single wavelength only
• D. no radiation at all

Explanation: A line spectrum arises because the electron can occupy only certain energy levels, so only certain energy differences, and therefore only…

• A. moves in a stable orbit
• B. jumps from a higher energy level to a lower one
• C. jumps from a lower energy level to a higher one
• D. is at rest in the nucleus

Explanation: Bohr postulated that an electron in an allowed orbit does not radiate at all, contradicting classical electromagnetism, and that a photon…

• A. minus 13.6 eV
• B. plus 13.6 eV
• C. zero
• D. minus 3.4 eV

Explanation: The negative sign means the electron is bound, and 13.6 eV is therefore the energy needed to remove it completely, which is the ionisation…

• A. n equals 1
• B. n equals 2
• C. n equals 3
• D. n equals 4

Explanation: Transitions down to the second level involve energy differences that correspond to visible photons, which is why Balmer lines are the ones…

• A. Infrared
• B. Visible
• C. Ultraviolet
• D. Radio

Explanation: Every Lyman transition ends on the ground state, so the energy released is large and the photons are ultraviolet.

• A. any value at all
• B. an integral multiple of h divided by 2 pi
• C. always zero
• D. equal to hf

Explanation: Quantising angular momentum in units of h over 2 pi is what restricts the electron to particular orbits and therefore particular energies…

• A. a hot gas emits light
• B. white light passes through a cooler gas, which removes photons of its own characteristic energies
• C. light passes through a vacuum
• D. a solid is heated until it glows

Explanation: Atoms in the cooler gas absorb exactly the photon energies that match their own energy level differences, leaving dark lines in the…

• A. n equals 1 to n equals 2
• B. n equals 2 to n equals 1
• C. n equals 1 to infinity
• D. infinity to n equals 1

Explanation: Ionisation means removing the electron completely, so it must be raised from the ground state to the level where its energy is zero, which…

• A. the difference in energy between the two levels
• B. the sum of the two energy levels
• C. the number of electrons in the atom only
• D. the temperature of the gas

Explanation: The photon carries away exactly the energy difference, and since energy is hc over wavelength, a larger gap gives a shorter wavelength.

• A. cannot explain the hydrogen spectrum
• B. works well only for hydrogen and other one electron systems
• C. assumes energy is continuous
• D. does not use quantisation

Explanation: The model reproduces the hydrogen spectrum beautifully but fails for atoms with more than one electron, and it cannot account for the…

• A. spontaneous emission only
• B. stimulated emission, in which a passing photon triggers an identical photon from an excited atom
• C. heating a filament
• D. the photoelectric effect

Explanation: The triggered photon matches the original in frequency, phase and direction, so the beam is coherent and highly directional, which is what…

• A. faster
• B. monochromatic, coherent and highly directional
• C. always more intense at every wavelength
• D. invisible

Explanation: A single transition supplies essentially one wavelength, stimulated emission keeps the waves in phase, and the resonant cavity makes the…

• A. an outer electron is excited
• B. an inner shell electron is knocked out and an outer electron falls in to fill the vacancy
• C. the nucleus decays
• D. electrons are slowed by the target

Explanation: The energy gaps between inner shells are large, of the order of thousands of electron volts, so the photons emitted lie in the X ray…

• A. proportional to n
• B. proportional to n squared
• C. inversely proportional to n squared
• D. independent of n

Explanation: The energy is minus 13.6 divided by n squared electron volts, so the levels crowd closer together as n rises and converge on zero at…

• A. every element has its own unique pattern of spectral lines
• B. all stars contain the same elements
• C. starlight is white
• D. the star's temperature is known in advance

Explanation: The line pattern is fixed by the element's own energy levels, so it acts as a fingerprint that survives the journey across space.

• A. 1.89 eV
• B. 13.6 eV
• C. 10.2 eV
• D. 3.4 eV

Explanation: The n equals 3 level is at minus 1.51 eV and n equals 2 at minus 3.4 eV, so the difference is 1.89 eV, a red photon and the first line of…

• A. the gas burns
• B. accelerated electrons collide with gas atoms, exciting them, and the atoms then emit light as they return to lower levels
• C. the glass becomes hot
• D. the gas is compressed

Explanation: Electrons accelerated by the field transfer energy to the atoms in collisions, lifting electrons to higher levels, and the light appears…

• A. the next level up
• B. n equals infinity, that is from a free electron
• C. the ground state
• D. a level below the final one

Explanation: As the starting level rises the lines crowd together and converge on a shortest wavelength, which corresponds to an electron falling from…

• A. reflect ultraviolet light
• B. absorb ultraviolet photons and re-emit the energy as several lower energy visible photons
• C. become hot
• D. emit ultraviolet light of their own

Explanation: The absorbed photon lifts an electron high, and it returns in smaller steps, so the emitted photons have less energy and longer…

• A. spectrum of the hydrogen atom
• B. spectra of all multi electron atoms
• C. structure of the nucleus
• D. photoelectric effect

Explanation: The model predicted the wavelengths of the hydrogen lines to remarkable accuracy, which is why it was accepted despite its…